1. Technical Field
The present invention relates to an electrophoresis device, an electronic apparatus, and a driving method of the electrophoresis device.
2. Related Art
An electrophoresis device is constructed by sealing an electrophoretic dispersion liquid containing one or more kinds of electrophoretic particles and an electrophoretic dispersion medium between a set of opposed electrode plates at least one of which is transparent. By applying a voltage between two electrodes, the electrophoretic particles move in the electrophoretic dispersion medium and the distribution thereof change accordingly. This changes optical reflection characteristics, enabling display of information.
In the electrophoresis device, it is necessary to apply a bipolar voltage between the two electrodes in order to move the electrophoretic particles reversibly, However, a transistor used for driving of the electrophoresis device has unipolarity.
As a technique for solving this problem, there is a technique disclosed in, for example, JP-A-52-70791. According to this technique, in an electrophoretic display panel, the potential of a pixel electrode divided into a plurality of segment electrodes is maintained at either of two different potentials V1 and V2 (V1<V2), and a pulse voltage which varies between V1 and V2 is applied to an opposed common electrode.
Thereby, when the potential of the common electrode is V2, an electric field is generated from the common electrode toward the pixel electrode in a region of the pixel electrode of potential V1, while an electric field is not generated in a region of the pixel electrode of potential V2. Therefore, if the electrophoretic particles are positively charged, the electrophoretic particles will migrate toward the direction of the pixel electrode in the region of the pixel electrode of potential V1, and the particles will not migrate in the region of the pixel electrode of potential V2. On the contrary, when the potential of the common electrode is V1, an electric field is generated from the pixel electrode toward the common electrode in the region of the pixel electrode of potential V2, while an electric field is not generated in a region of the pixel electrode of potential V1. Therefore, positively charged electrophoretic particles migrate toward the direction of the common electrode in the region of the pixel electrode of potential V2, and any particles do not migrate in the region of the pixel electrode of potential V1.
By changing the potential of the common electrode at least one or more cycles between V1 and V2 in this way, the electrophoretic particles can move alternately in the region of each pixel electrode, and consequently the electrophoretic particles of each region can be migrated toward a desired direction. According to this method, since the voltages applied to the common electrode are only V1 and V2, it is also possible to use a unipolar transistor.
However, the above method has a problem in that, since the voltage to be applied to the pixel electrode shifts due to factors, such as a voltage drop by wiring resistance and leak, display may be disturbed. That is, not only V1 and V2 but also the potentials V3 and V4 shifted from V1 and V2 under the influence of wiring resistance, wiring capacity, and leak, appear actually in a pixel electrode. Here, a case in which V3 is slightly higher than V1 and V4 is slightly lower than V2 will be described. Since wiring lines on the side of the pixel electrodes generally are formed as minutely as possible in order to increase the density of pixels, a voltage drop by wiring resistance and the voltage shifting by leak are apt to occur. On the other hand, since wiring lines on the side of the common electrode is relatively sparse and thick wiring lines are allowed, a voltage drop by wiring resistance and voltage shifting by leak occur hardly.
In this case, when the potential of the common electrode is V2, the relationship V3<V2 is established in the region of the pixel electrode 13a-1 of potential V3. Therefore an electric field is generated in the direction of the pixel electrode, and if electrophoretic particles are charged positively, the electrophoretic particles migrate toward the direction of the pixel electrode On the other hand, since the relationship V4<V2 is established also in the region of the pixel electrode of potential V4, an electric field, though slight, may be generated in the direction of the pixel electrode, Further, when the potential of the common electrode is V1, the relationship V4>V1 is established in the region of the pixel electrode of potential V4. Therefore, an electric field is generated in the direction of the common electrode, and electrophoretic particles which are charged positively migrate toward the direction of the common electrode. On the other hand, since the relationship V3>V1 is established also in the region of the pixel electrode having potential V4, an electric field, though slight, may be generated in the direction of the common electrode. Since the electrophoresis device does not have threshold characteristics, the electrophoretic particles may migrate also in response to such slight electric field, which causes deterioration of display quality.